This codec is assembled for the MIMXRT1xxx_DEV boards and available
for WM8960 breakout boards as well.

The driver itself has been tested as working with the MIMXRT boards
and a Sparkfun WM6890 breakout board. It implements the initialization,
basic methods and some enhanced methods like 3D, ALC or Soft Mute.

Following the suggestions of @dpgeorge:
- create a Regs class, which encapsulates accesses to the hardeware
  and holds the register cache.
- Change WM8960 register accesses into looking like array accesses.
- Combine modifications of the same register into one operation.
- Put assignments to L/R registers with the same value into one line.

Black had to be told to leave the code untouched, because it shifted
the whole WM8960 class.

This array is initialized by a tuple with the power-on values.

Which also made a few register shift definitions obsolete.

- remove unneded statement in set_volume()
- masked additional bits in get_volume()
- rewrote a line in __setitem__()
- shortened register access in three functions.

The prefix drop seems to change about every line of code. So it looks
like a larger change than it is, but the code looks better.

- Normalize volume setting to the range 0-100.
- Use a table to simplify the code.
- Reformat the documentation.
- Add a default value for i2c_address.
- Some reformatting made by black.

According to the suggestion of @dpgeorge.
Fix a few omissions of the previous commits as well.

Still there is a warning about the wm8960.rst document not contained
in any doc tree. What would be the best place?

Silencing the warning about a missing toctree entry.

For some reason, this is required for one of my boards, and does not
hurt on the other ones.